WEED CONTROL AT MNSC


This page is included in view of the interest in our weed control methods, developed following the proliferation of Canadian Pondweed in our waters.

Background
We sail on a flooded gravel pit, about 140 acres in area, with a depth of about 8 – 10 feet over most of the area. The lake is adjacent to the River Nene, and seepage from this together with the presence of springs, means the lake cannot be drained or pumped out. Although we own and have full access to the surrounding banks, the uneven ground and presence of trees and shrubs prevents the use of heavy, land based equipment.

Between 1946 and 1994 we experienced no problems with weed. In 1994 – 1995 significant growth of Elodea Canadensis/Elodea Nuttali was apparent. Since then we have considered various control methods, chemicals, covering the lake bottom with plastic, weed eating snails, grass eating carp, cutting, rotavating, etc., etc. Most were ruled out on cost, environmental, or reliability grounds, leaving chemical and cutting as deemed worth trying.

Chemical Control
In 1996 a small scale trial of the recommended chemical weedkiller for our situation (dichlobenil) was undertaken. This proved ineffectual and, combined with the high cost to treat the whole of our lake (about £28,000 per year) caused us to reject this and consider other means.
(Further trials in 2001 and 2002 over an area of 1 hectare gave the same result, no discernable difference between treated and untreated areas.)

Mechanical Control
Cutting seemed the best alternative, and from 1997 to the present this has been our favoured control method. The process has been refined and speeded up over the years, and we can now complete a full and accurate cut of our lake in 12 hours only, using a 6.5 metre wide "V" blade towed at 8 kph using an on-board GPS receiver to follow a pre-mapped cutting path.

Our weed cutting experience started in 1997 when we invested in a reconditioned Conver weedcutting boat. This incorporates a front mounted reciprocating cutter that allows weed to be cut to a depth of about 1 metre below the surface, and, alternatively, can tow a “V” blade to allow cutting on the bottom if weed and bottom conditions are suitable. The V blade is towed via a reciprocating linkage, to ensure the blade alternately sinks solidly onto the bottom, and is then jerked forward fast enough to cut effectively. An augur drive, instead of a normal propellor, allows operation in weed-choked water.
We quickly found that the front mounted cutter was not successful, as it did not cut deeply enough and the weed quickly re-grew. However, the towed V blade was very effective, and we could cut a swathe 2.5 metres wide at an average speed of about 4 k.p.h. (Theoretically, this meant we could cut all of our main sailing area in about 60 hours.)
But there was a major difficulty. We cut when the weed became a problem, i.e. mid season. The cut weed floated, resulting in our lake being covered with vast, floating rafts of impenetrable, compacted weed. Attempts to clear this, using the front mounted forks on the Conver proved unacceptably slow and laborious.

The answer was to cut during the winter months when the cut weed sank and decomposed. We found that a winter cut gave us freedom from weed the following season, as the weed did not have time to make sufficient growth to inhibit sailing. This worked well, but highlighted the next challenge, how to cut an area of 100 acres plus, accurately, and without missing anywhere, when you can't see the bottom. A system of transits was devised which was a vast improvement on “blind” cutting.  We had two years of reasonable success using transits, with weed-free sailing seasons following winter cuts.

However, by 2000, we were getting sufficient re-growth by about July/August to cause major problems. In 2001 we attempted to address this by purchasing a used “Miller Harvester” to collect floating weed previously cut by our Conver. The Miller Harvester utilises a conveyor belt 1.8 metres wide to continuously collect cut weed, and transport it into the hull of the vessel. When full, the conveyor can be raised and reversed so that the weed can be unloaded directly on to a suitable bank. We found that although the system works well, it is very slow when attempting to clear large areas of water some distance from the banks. About 1 acre per day was the best that we could achieve. However, because of varying wind directions during the collecting period, weed is continually being blown into into previously cleared areas, so in fact complete clearance was never possible. An attempt to boom off areas of floating weed by using a floating (polypropylene) rope was not successful and was inconvenient.

Click on an image to enlarge, and use "back" button on browser to return to this page



In 2000, 2001, and 2002, weed growth restricted our useable sailing area in the latter part of the season, which was not acceptable. Although it was initially thought the mid-season problem was caused by re-growth from weed plants cut the previous winter, a suspicion was developing that the growth was from areas missed in the cut. It was known that the transit system is not that precise, particularly when the transits are distant and behind the boat. So we looked for ways to improve the cut accuracy.

The two best options seemed to be laser guidance or a GPS mapped path.  The laser would be more accurate, but would need someone on the shore to move the source or reflector after every run, tedious in the extreme!  So the GPS was decided on, and many happy hours were spent plotting the required 890 way points to cover our lake. However once done, it's done!  We tested the system in April, 2003 over a 30 acre area, the remainder having been cut using our normal transit method. At the end of October, 2003, the GPS guided cut area was weed free, the transit cut remainder had numerous clumps of weed reaching the surface. Between November, 2003 and February 2004, we cut the whole lake area using the GPS system, and were weed-free over the whole area for the following year. The procedure was repeated in the 2004/2005 winter, again with a successful result and weed free sailing the following year.



Click on an image to enlarge, and use "back" button on browser to return to this page


Then came the winter of 2005/2006 which was unseasonably warm. The weed continued growing immediately after cutting, and was already reaching the surface in June, with the result that we lost the use of about half our lake area. We had to assume that the warm winter was likely to become the norm, so a way had to be found of dealing with the new situation.

We felt that cutting was still the only practical option. But to be viable, the cutting would have to be so frequent that the weed would never attain a length of more than a few inches, or a way would have to be found of dealing with the floating rafts of weed that would otherwise cover the surface. So rapid, frequent cutting seemed the best possibility.

The Aims
The existing Conver cuts a swathe 2.5 m wide at an average speed of 4 k.p.h., and takes about 70 hours to complete one full cut of the lake. Assuming (optimistically!) volunteers would man the Conver for 18 hours/week this would still have meant that four weeks would elapse between consecutive cuts in the same place. Meantime, with an observed weed growth rate of over 1 metre/week, the weed could have grown 4 metres, meaning the Conver would leave a trail of 4 m long Elodea floating on the surface as it repeated its cutting path. This would immediately scupper any possibility of sailing.
However, if we could significantly reduce the cutting time, continual cutting becomes a possibility. So the initial aim was set to develop a means to complete a full cut in 18 hours, i.e. four times as fast as the Conver.

The project started with a search for a suitable craft to tow a larger V blade, and after viewing a number of possibilities, we found a Beaver inshore tug boat that seemed to fit the bill. Our offer was accepted, and the Beaver was delivered on October 12th, 2006.
The Beaver was refurbished over the 2006/2007 winter, fitted with a reciprocator, and a new, much larger, V blade. We decided to start with a width of 6.5 m, which could then be shortened if necessary. The blade section was calculated to have a similar maximum allowable stress as the 2.5 m blade of the Conver.
Initial tests worked quite well, and following some experimentation with chain lengths we found we could tow at up to 6 k.p.h. before the blade “flew” but it was apparent the reciprocator was over stressed, and various failures occurred.

At this stage we decided to try without the reciprocator. As expected, the blade “flew” at speeds over 4 k.p.h., which is too slow anyway for effective cutting. (The principle of the reciprocator is that it enables the boat to travel below the critical “lift off “ speed, but the reciprocator action alternately increases the blade speed to 8 kph giving an effective cut and then reduces it to zero so the blade sinks ready for the next cut.)

So the challenge was to find a way to keep the blade on the lake bed when the speed was the 8 k.p.h. necessary for good cutting. Following many hours of experimentation, first on the 2.5 m Conver blade, and finally on the 6.5 m Beaver blade, we found the combination of chain length and blade negative incidence that reliably kept the blade on the bottom at the optimum cutting speed of 8 k.p.h.

The GPS way points were then re-plotted to give cutting path centres of 4 m, allowing 2.5 m overlap between adjacent tracks.

The Beaver went into operation in June, 2007, and soon demonstrated its capability to cut quickly and effectively. It has proved possible to follow the cutting tracks very accurately (more accurately than the Conver), and the initial aim of a full cut in 18 hours has been comfortably bettered, and in practice a full cut is now taking only 12 hours.


Click on an image to enlarge, and use "back" button on browser to return to this page


Collecting/Macerating Development
Even with a much imcreased frequency of cut, it was anticipated that some patches of cut weed would surface and be a nuisance, so consideration was given to increasing the rate the Miller Harvester (Manatee) could deal with floating, cut weed. The biggest problem with the Manatee was the time it took to transport a load of collected weed to the bank, unload it, and return to the cutting area. So we looked into the possibility of macerating the weed as it was collected, and returning it directly to the lake as some sort of “sludge”. Various options were considered for the macerating process, but the immediate availability of an agricultural forage harvester meant this was tried first. Coupling it up to a tractor PTO and throwing in armfuls of fresh Elodea showed we were on the right track, and so we went ahead with installing the forage harvester in the Manatee. Both the forage harvester and the Manatee required considerable modification, but the final result, although maybe not the prettiest of craft, works well. The modified Manatee also went into operation in June, 2007, and and does a very impressive job of collecting patches of floating Elodea and spitting it out as a green sludge, which rapidly disperses in the water, exactly as it was designed to do.


Click on an image to enlarge, and use "back" button on browser to return to this page


Although weed growth rate in 2007 was equal or possibly greater than in 2006, our sailing and racing in 2007 was unaffected by weed. The frequent cutting now possible with the Beaver kept the weed under control and meant it never got close enough to the surface to be a problem, and the few patches of floating cut weed that did appear were quickly dealt with by the Manatee.
We are now waiting to see the next challenge Elodea Canadensis will throw at us!
We would be very interested to hear of other Clubs experience with their weed control methods.

Useful Contacts
Reedman Services - Weed cutting boats (new and used) spares, good advice.
www.reedmanservices.co.uk  phone : Andrew 01427-787810
 
Dr. Jonathan Newman - The acknowledged guru on Elodea!
(Head of Centre, Rothamsted Research, Centre for Aquatic Plant Management)